DESCRIPTION: Approximately seven million Americans are currently using calcium channel blockers primarily for treatment of hypertension or angina. To date, his class of drugs has not been evaluated for long-term outcomes, including incident heart disease and cancer. Two prospective cohort studies of the elderly have found an association between use of calcium channel blockers and incident cancer, one, site-specific to breast cancer (hazard ratio=2.57, CI=1.47-4.49). The intent of this study is to examine further the relationship between the use of calcium channel blockers and incident breast cancer in women, aged 55-84, enrolled in the Group Health Cooperative (GHC), an HMO based in Seattle, WA. The risk of breast cancer will be assessed according to ever-use, dose, duration, and chemical class of calcium channel blockers. An historical cohort design will be used to initially examine this relationship; a nested case-control study will be included to assess potential confounding and effect modification by other variables. The cohort will consist of all women, aged 55-84, enrolled in GHC on January 1, 1988 (approximately 37,300 women). The cohort will be followed through 1996 for incident breast cancer identified through the Cancer Surveillance System. Approximately 225 cases per year are expected. Exposure will be assessed from the GHC computerized pharmacy database. Data on use of other medications, pathology of the tumors, age, and inpatient diagnoses and procedures are available to evaluate confounding and effect modification on the cohort. A nested case-control study will supplement the cohort analysis by assessing data not available electronically and to determine the extent of misclassification in the cohort analyses. Four-hundred hypertensive cases will be frequency matched to 400 hypertensive controls, for whom medical records and abstraction of the following data will be done: age, race, parity, body mass index, smoking history, family history of breast cancer, medication use (including hypertensives and estrogen replacement therapy), history of previous diagnoses of CHD, previous cardiac procedures and mammography.